Sequence-paleogeography and coal accumulation of the early and middle Jurassic in the Junggar Basin

Analysed the sequence stratigraphy and palaeogeography of the early and early middle Jurassic coal measures in the Junggar Basin,northwestern China.The controls of the palaeogeography and sequence stratigraphy on the coal accumulation in this basin was discussed.The results show that,the coal measures were formed in a fluvial-delta-lacustrine sedimentary system,and a total of 3 third-order sequences were developed,corresponded to Badaowan Formation(J1b),Sangonghe Formation(J1s) and Xishanyao Formation(J2x).The Junggar Basin had experienced the swamping(Sequence I)—lake flooding(Sequence II)—swamping(Sequence III) processes.The thick coal seams were developed at the late transgressive systems tract and the early highstand systems tract and near the maximum flood surface,when the rate in creation of accommodation kept balance with the peat accumulation rate.Sequence I and sequence III's coal-forming environments are floodplain swamp,the delta plain and the shallow lake,especially delta plain best.